The present invention relates to a data processing apparatus and, more particularly, to a bus arbitration system for a data processing apparatus to which a plurality of bus masters of relatively low degrees of priority are connected.
Conventional bus arbitration systems for the above application are disclosed in, for example, Japanese Patent Application Disclosure No. Sho 62-229353.
The above document discloses three different conventional systems, i.e., a first system using a daisy chain system, a second system using a priority arbitration system, and a third system using a round robin system. The daisy chain system connects a plurality of bus masters in series and gives the highest priority to the electrically uppermost bus master. The priority arbitration system connects a plurality of bus masters in parallel, inputs to a bus arbiter bus request signals which are outputted by the bus masters in synchronism, and sequentially selects the bus masters according to a predetermined priority order to give them an allowance signal. Further, the round robin system causes a bus arbiter to cyclically scan bus masters to see if any one of them is outputting a bus request signal, give an allowance signal to the bus master outputting a bus request signal on detecting such a bus master, and handles this bus master as a bus master of lowest priority at the time of the next bus arbitration.
The above-described conventional bus arbitration systems have the following problems.
To begin with, a problem with the first conventional system is that the arbitration processing time increases with the increase in the number of bus masters. Another problem is that the priority order among the bus masters is fixedly determined by their connecting positions.
The second conventional system is disadvantageous in that since the priority order for dealing with the request signals from the bus masters is predetermined, an allowance signal is given only to a bus master having high priority.
Regarding the third conventional system, assume that a plurality of bus masters are far different from one another with respect to the duration of bus occupation. Then, when an allowance is given to the bus master which occupies the bus over the longest period of time, the bus arbitration is partial from the standpoint of bus occupation ratio.